1. Field of the Invention
The invention is based on an electric resistor having a resistor core which is arranged between two contact terminals and contains a material which has a PTC behaviour and, below a material-specific temperature, forms at least one electrically conducting path running between the two contact terminals.
2. Discussion of Background
A resistor of the type mentioned above has long been state of the art and is described, for example, in DE 2 948 350 C2 or U.S. Pat. No. 4,534,889 A. Such a resistor contains a resistor core made of a ceramic or polymeric material which exhibits PTC behavior and, below a material-specific limiting temperature, conducts electric current well. PTC material is, for example, a ceramic based on doped barium titanate or an electrically conductive polymer, for instance a thermoplastic, semicrystalline polymer, such as polyethylene, with for example carbon black as conductive filler. If the limiting temperature is exceeded, the resistivity of the resistor based on a PTC material increases abruptly by many orders of magnitude.
Therefore, PTC resistors can be used as an overload protection for circuits. On account of their restricted conductivity, carbon-filled polymers, for example, have a resistivity greater than 1 .OMEGA.cm, they are generally restricted in their practical application to rated currents up to about 8 A at 30 V and up to about 0.2 A at 250 V.
Specified in J. Mat. Sci. 26(1991) 145 et seq. are PTC resistors based on a polymer filled with borides, silicides or carbides having a very high conductivity at room temperature which are said to be useable as current-limiting elements even in power circuits with currents of, for example, 50 to 100 A at 250 V. However, such resistors are not commercially available and therefore cannot be realized without considerable expenditure.
In the case of all PTC resistors, the thickness of the resistance material between the contact terminals, together with the dielectric strength of this material, determines the magnitude of the voltage held by the resistor in the high-impedance state. In the case of a rapid transition from the low-impedance state to the high-impedance state, however, larger overvoltages are induced--in particular in the case of circuits with high inductance. These overvoltages can only be effectively reduced if the PTC resistor is given large dimensions. This inevitably leads either to a considerable reduction in its current-carrying capacity or to an unacceptably large component. In addition, it may happen that, in the case of overloading at locally predetermined points, such as for instance in the center between the contact terminals--the PTC resistor becomes hotter than at other locations and consequently switches into the high-impedance state earlier at these points than at the non-heated locations. Then the entire voltage applied across the PTC resistor drops over a relatively small distance at the location of the highest resistance. The associated high electric field strength may then lead to disruptive discharges and to damage of the PTC resistor.